Spiral galaxies are like people: they fray as they age. As people gradually sag and wrinkle, so too a young galaxy's elegant spiral structure frays into myriad substructures-spurs, feathers, and filaments-branching off from the main spiral arms. These substructures interfere with the needs of advanced life.
How? Bright stars and dense molecular clouds are plentiful in spiral arms and in substructures. These stars and clouds would tend to destabilize the orbits of whole planetary systems or of the individual planets within them. Such destabilization would make advanced life impossible, and so would the shower of radiation from these bright stars.
However, in spiral galaxies young enough to be unfrayed, life-essential elements are missing, specifically the heavy elements such as uranium and thorium. The appropriate abundances become available only when a galaxy reaches the not-so-youthful age of 9-10 billion years.1
The way around this substructure challenge is to find a sufficiently aged spiral galaxy with minimal fraying and then locate a just-right (in hundreds of ways) planet in a region least impacted by fraying. Such a location is exactly where Earth finds itself. An amazing coincidence?
Recent discoveries shed new light on just how amazing.2 Research teams have found that the fraying process is intricately complex, affected by multiple galactic parameters. To keep the fraying within the acceptable range for advanced life, the galaxy's magnetic field must be relatively weak, yet strong enough to prevent the spiral's collapse. Its disk must be dense enough but not too dense. And the quantity of gas in the spiral arms as well as the differential compression of gas flowing through them must be relatively low, yet high enough to sustain the spiral structure.
Similar precision is required for the location of any advanced-life site within a minimally frayed galaxy. It must reside near what's called the corotation distance (the distance at which stars revolve around the galactic core at the same speed as the spiral arms, along with their substructures, rotate). A team of researchers at the University of Maryland recently observed that near the corotation distance, substructures part pathways, leaving a small gap like a part in one's hair. So from the fraying standpoint also, the best place for an advanced-life-support planet is near the corotation distance.3 And that's where we are.
The fact that advanced life requires so much precision of its galaxy and galaxy features and of its position within that specialized galaxy makes our Milky Way Galaxy seem all the more remarkable. The data indicate Someone intended for advanced life to be here.